The present invention relates to improvements in rotary drying systems, and more particularly to an improved hog fuel dryer wherein contamination of the fuel such as rocks and foreign material are removed so that they will not be transported through the dryer and cause damage.
Dryer systems are used for many purposes, including drying sludge prior to disposal, drying crops prior to further processing and drying fuel prior to combustion.
Fuel drying systems are widely used, and the interest in such systems results from the proven fact that dry fuel increases boiler efficiency. Drying of the fuel prior to being supplied to a boiler is highly advantageous in that if the drying is done inside the boiler, some of the energy which could be used to generate steam is instead consumed to dry incoming fuel. Boilers are inefficient dryers and the effect of wet fuel on the boiler performance is dramatic. Drying fuel in advance of the boiler results in more available BTU's per pound of fuel, as well as increased boiler efficiency. Drying also allows the effective control of fuel moisture content as moisture varies by season and with the composition of raw material. The consistency of the fuel moisture also contributes to more efficient and predictable boiler performance.
A commonly used fuel dryer operates on the rotary principle, wherein a rotary drum is fed with the fuel, and heated gases remove moisture from the fuel. Such a dryer can increase boiler efficiency by 10% or more. For example, if fuel is burned at a 55% moisture content in a boiler with a 500.degree. F. flue gas temperature, drying can be conducted to substantially reduce the moisture to combustion sustaining levels. If the moisture content of the fuel is reduced from 55% to 33%, the boiler efficiency will increase from approximately 57% to about 68%, thereby increasing boiler efficiency by more than 10%.
Rotary dryers are used commonly to dry fuel in industries such as the pulp and papermaking industry. In such industry, naturally occurring fuel, such as hog fuel, is widely available, and such fuel often becomes contaminated with rocks and foreign material. If the rocks and foreign material are permitted to enter a rotary dryer, extensive damage can result to flights and other internal components of the dryer. Since the material being dried is carried, at least in part, by the airstream through the dryer, the foreign elements, which are often much heavier than the fuel, cannot otherwise get out of the drum, due to the nature of the dryer construction. The foreign material tumbles the length of the dryer or concentrates in one area, in either case being tumbled at length by the dryer vanes or flights. Large or heavy foreign material, such as rocks, can cause significant damage, while being tumbled in the dryer.